[jeff5may]

With residential AC units and most heat pumps, the indoor and outdoor coils ARE the liquid receivers. If the unit is a smaller, cap-tube metered setup, it will probably have a "bullet" filter/dryer on it right before the cap tube, which kind of serves the same purpose. Medium-sized portable/window units have a section of each coil where the split flow merges, generally at the bottom of the coil. The section is situated a couple feet worth of piping from the compressor side fitting. Also, there is space at the bottom of the compressor that acts as a catch-can for oil or liquid slugs.

As the units become larger and split, around 2-1/2 to 3 tons of capacity, expansion valves and suction accumulators begin to enter the picture. The manufacturers have deemed units smaller than this size not worthy of these luxuries. On some heat pump units, manufacturers have plumbed in a "charge compensator" (aka "quiggle-ator") to try to minimize liquid refrigerant from overly backing up the condenser coil during heating season. Compared to commercial rigs, none of these components are very large or high-capacity.

Very few residential units have employed multiple compressors. Rather, the mfr's have come up with a few good designs and fed the masses with them. Air-source units in general have evolved into largely maintenance-free devices, so they would rather not mess a good thing up by changing time-tested designs. The public will not stand for a unit that requires any real preventive maintenance.

One such design is the twin-single compressor. It basically has two rotary/scroll assemblies in the pot, like a clothes washer. On low speed, one of the assemblies rotates while the other freewheels. On high speed, the motor reverses direction and both assemblies rotate. Works like a champ.

Another predominant setup is the variable-speed, or inverter drive compressor. After a slow start, this design is beginning to show up in lots of new units. Apparently, it took awhile to convince the industry that they actually work well.

The auto industry in general went with the variable displacement compressor first. The residential mfr's have been experimenting with the idea, but AFAIK, nothing has made it to mass production.

The multiple-compressor idea has pretty much been abandoned by the residential HVAC manufacturers. Oil and refrigerant flow controls are needed, due to the multiple refrigerant paths. Additional electronic controls are needed to switch and start multiple compressors. More copper plumbing is needed for the refrigerant circuit. All these extras incur costs that the mfr's must consider, both for building and providing service after the sale.

In the commercial sector, customers care about serviceability, redundancy, and ROI. Businesses have no problems paying extra for a system that will run at reduced capacity while they swap a burnt compressor, or that will save at least X amount of dollars a month running on low. Not so much in the residential market. Joe Snuffy wants a cheap, bionic unit that will live at least a week longer than he will, that's all. This formula adds up to the most basic, foolproof design possible at a certain level of performance. And no more.